Document U.S. Pat. No. 5,291,670 disclosed an oven for enamelling conductive wires coated in a layer of resin (or varnish) dissolved in solvents. The oven includes a horizontal main chamber of elongate shape, an auxiliary assembly for sucking in and treating the solvent vapors coming form the main chamber, and a system for hot air convection therein.
The main chamber of the oven is subdivided into a zone for evaporating the solvents form the layer of resin on the wires, and a zone for polymerizing and cross-linking the resin. It is fitted with a horizontal row of electric resistance elements, which are mounted inside the chamber, and with heat exchangers which define two of the opposite longitudinal walls of said main chamber.
The auxiliary assembly has a suction mechanism for sucking in a flow of air and solvent vapor from the main chamber, between the two zones thereof. It also includes a heater mechanism for treating the sucked in flow. It is coupled to one of the ends of the heat exchangers to feed them with hot air provided by the treated flow.
The hot air convection system has a duct which couples the other ends of the heat exchangers to one of the ends of the main chamber. This ensures that the treated flow is recycled, with the flow being reinserted into the main chamber after it has travelled through the heat exchangers along the chamber.
In the main chamber of that enamelling oven, the solvent evaporation zone is maintained at a temperature in the range 150.degree. C. to 350.degree. C. while the resin polymerization and cross-linking zone is maintained in the range 400.degree. C. to 550.degree. C. The flow sucked in from main chamber is treated at a temperature of about 700.degree. C. to 750.degree. C. in the auxiliary assembly. Each of the tow zones in the main chamber is maintained at the desired temperature by the heat delivered form the internal electrical resistance elements, and also to a large extent by the heat exchangers and the convection system.
The enamelling oven makes maximum use of heat exchange between the treated flow fo air and solvents and the wires to be enamelled. Nevertheless, it is complex and bulky in structure.
Document JP-A-01 11671 discloses an assembly for baking a layer of resin (or varnish) on a metal wire, comprising two heating ovens which are mounted one after the other and through which there travels the wire, coated in its resin layer. The first heating oven is a circulating air oven. It ensures that most of the solvents in the layer of resin on the wire evaporate and it maintains resin cross-linking at an insufficient degree, with oxidation of the resin and of the wire remaining below a certain degree. The second heating oven causes the resin to be cross-linked at high temperature, of an the order of 500.degree. C., and is provided with an internal atmosphere having low oxygen content, less than 5%, to hinder oxidation of the resin and of the wire. The enamelling assembly requires a unit for treating the solvent vapor coming from the first heating oven, if pollution problems are to be avoided.